A conventional current mirror circuit using MOS transistors is shown in FIG. 1. The circuit operates from a V.sub.DD power supply with its positive terminal connected to terminal 10 and its negative terminal connected to ground terminal 11. An input current I.sub.IN passing through p-channel transistor 14 and to terminal 12 is reflected as I.sub.OUT flowing as an output current through p-channel transistor 15 and to terminal 13. Transistor 14 is diode-connected with its gate connected to its drain. The gate of transistor 14 is connected to the gate of transistor 15. With such a configuration, transistor 14 develops a threshold voltage drop, V.sub.T, which is applied to the gate of transistor 15 thereby causing it to conduct I.sub.OUT. If the two transistors are matched, then I.sub.OUT equals I.sub.IN. By varying device parameters, the transistors can produce either a current gain or loss. Typically, the transistors are fabricated to have the same channel length, while the channel widths are varied to provide channels having different areas and thereby passing different currents. Thus, the channel width is a design factor in circuit performance. While FIG. 1 shows a single output transistor, a plurality of output transistors can be coupled to the input transistor so that a single input current can be mirrored as a plurality of related output currents. The various output transistors can then be sized to produce the required current values.
In addition, although FIG. 1 shows p-channel transistors sourcing currents, if desired, current sinks can be created using n-channel transistors connected in the same manner to complement the p-channel device configuration. Clearly, both polarities can be employed in CMOS structures.
Ideally, an operational amplifier (op amp) is a differential input, single-ended output voltage amplifier that provides infinite voltage gain with infinite input impedance and bandwidth and zero output impedance.